SmartPower

Product: SmartPower
Website: http://ignatu.co.uk
Type: Free
Requirements: Windows XP/Vista/7

SmartPower is a Windows service that saves energy (and money) by hibernating and resuming your Windows PC according to configurable rules. SmartPower is particularly suited to Windows Home Servers, torrent/download boxes and HTPCs.

How does it work?
SmartPower's "goal" is to hibernate (or suspend, or shutdown) your PC. The SmartPower configuration utility lets you create rules that prevent this happening. When SmartPower sees that no rules currently apply it will initiate hibernate (or suspend, or shutdown).